The present invention relates to circuitry for processing electrical signals, and more particularly to circuitry for changing the frequency of such signals to enhance their subsequent further processing and analysis.
Over the years, numerous instruments have been developed for measuring physical phenomena by detecting energy generated due to the phenomena and converting the energy to electrical signals. The electrical signals are processed to obtain information on a wide variety of physical characteristics, e.g. the location, size and velocity of an object, or its index of refraction, transmissivity or surface characteristics. In many applications, signals occur intermittently and randomly and vary in their amplitude and duration. Further, the signals may be expected to occur over a wide frequency bandwidth in which frequencies near the upper end of the bandwidth are at least several orders of magnitude greater than frequencies at the lower end.
While such applications can involve radar and sonar, the present invention is more directly concerned with laser Doppler velocimetry (LDV), also known as laser Doppler anemometry. LDV systems are known for their utility in measuring instantaneous velocities of discrete elements in two-phase flows, e.g. liquid sprays in air, or fine particles in fluid streams. In a conventional LDV apparatus, one or more pairs of spatially separated laser beams intersect one another and interfere with one another to form a measuring volume. The measuring volume is typically quite small, and the concentration of particles sufficiently low, so that at any given time no more than one particle is within the measuring volume. One or more photodetectors receive the coherent light scattered by particles passing through the measuring volume. The Doppler frequency, obtained by measuring the electrical signal generated as a function of light received by the photodetector, measures particle velocity and thus the velocity of the particle-carrying medium. Doppler frequency is proportional to particle velocity.
Laser phase Doppler systems are closely related to LDV systems and employ a phase difference between two separate Doppler frequency signals to determine the size of a moving particle. More particularly, two or more photodetectors receive light scattered by a particle passing through a measuring volume. The photodetectors receive light from different locations relative to the measuring volume. The difference in phase between signals from the photodetectors provides the particle size information.
Particles, especially spherical particles, tend to scatter light in all directions and thus lend themselves well to analysis by laser Doppler techniques. However, physical characteristics of systems under analysis can lead to problems in analyzing the resultant electrical signals. Given the typically low particle density and small size of the measuring volume, particles traverse the measuring volume individually and in intermittent, random fashion. The resultant electrical signal is a composite of background noise and occasional coherent frequency components, known as "bursts" superimposed on the background noise. Thus, signal processing circuitry should be capable of distinguishing the coherent frequency bursts from noise to avoid wasteful attempts to analyze the noise. The coherent frequency bursts tend to be non-uniform in amplitude, frequency and duration. Signal amplitudes vary with particle size, but also with varying tendencies of particles to absorb rather than scatter the laser energy. Signal frequencies can vary over orders of magnitude, particularly in turbulent flow systems. The length or duration of the bursts can vary considerably, even under conditions of uniform particle size and velocity, depending upon whether a particle is substantially centered as it traverses the measuring volume.
Known processing techniques, e.g. involving LDV counters, are capable of processing coherent frequency bursts in real time. However, the signal is prone to distortion and noise that depends on signal frequency, amplitude and processing bandwidth. Thus, the nature and degree of distortion is difficult to predict.
Alternatively, the electrical signals can be sampled and converted to corresponding digital signals and then processed digitally. While digital sampling and processing do not eliminate distortion, the distortion is more predictable. Typical digital processing techniques include signal correlation and fast Fourier transform. A disadvantage of this approach is that sampling and processing time can considerably exceed burst durations, seriously decreasing the rate at which the burst signals can be processed.
Given the wide range of possible burst signal frequencies, circuitry for processing the signals must be capable of functioning over a broad frequency band. This requirement calls for circuitry which is more complex, more costly and less reliable than circuitry tailored to handle a narrower frequency bandwidth.
To address this difficulty, analog and digital processors have been used to heterodyne, or translate, the signal frequency. Typically, analog mixing techniques are employed to limit the frequency range of signals subject to further processing, thus to reduce the complexity of processing circuitry. Analog mixing, however, is subject to reduced signal-to-noise ratio due to mixer insertion loss and post-mixer amplification and filtering. Intermodulation distortion and harmonic distortion also increase noise.
Therefore, it is an object of the present invention to provide circuitry for real time processing of analog signals, while minimizing noise and signal distortion.
Another object of the invention is to provide a process for rapidly adjusting the frequency bandwidth of electrical signals before further processing of the signals.
A further object is to provide a means for receiving signals over a broad range of frequencies and conditioning the signals for further processing within a considerably narrowed frequency bandwidth.
Yet another object is to provide simpler, less costly and more reliable circuitry for frequency translation of coherent frequency bursts.